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Three years ago, in a local body election year, the Prime Minister did an about-turn on an Auckland underground rail link, endorsing the project in principle though making no financial provision for it. Yesterday, he endorsed the Auckland Council's plan to go ahead with the project in 2018, the Government's financial contribution to be based on a business plan that will be devised in co-operation with the council.

It is, of course, another local body election year. Clearly Mr Key did not want the central rail loop to be an issue that could decide the political complexion of the next mayor and council.

There is no other credible explanation for the decision. The economy is hardly in such a state that it needs an infrastructural shot in the arm, certainly not in Auckland. And the central rail link still fails to justify its cost to the country's transport planners against other national priorities in their budget. But then, officials in Wellington have never been convinced by Auckland transport planners that rail is a solution for this city.

The Prime Minister's reading of the politics of the issue is an implicit tribute to the effort of the departing mayor, Len Brown, as it was three years ago. By the end of his first term, Mr Brown had pumped so much enthusiasm into the long-languishing rail scheme that it almost felt inevitable. That momentum has been sustained despite the mayor's fall from grace, in large part by his council's decision to start the first stage of the work on the still unfunded project so redevelopment of the downtown site can proceed.

To the mayor's credit, he put several alternative funding options in front of Auckland voters, none of them likely to be popular. None of them were palatable to the Government either. Road tolls, congestion charges, a regional petrol tax, an inner city parking levy; all would have been an incentive to use public transport, as well as a fair charge on remaining drivers who would benefit from reduced road congestion - so long as the rail scheme worked. If the charges had been put to voters, the proposition would have been a good hip-pocket test of Aucklanders' faith in railways.

Mr Brown expresses complete faith that the central link is the last piece in the jigsaw, enabling trains on all three lines through the isthmus to turn on an inner city circuit rather than terminate at Britomart. It should certainly enable trains to run more frequently, maybe more than doubling the services from 20 to 40 trains an hour through the station. Additional stations will also be built under the CBD.

The central loop will not cure all problems for commuter rail in Auckland. A solution to suburban level crossings that slow trains may be the next expensive demand. But the $2.5 billion underground link should be a vast improvement. It is the realisation of dreams that date from early last century. The retiring mayor can claim an achievement that eluded Sir Dove-Myer Robinson and languished on regional transport plans for the next 40 years. The amalgamation of local government in 2010 enabled Auckland to speak with one voice, but nobody could deny the vocal efforts of the Super City's first mayor have been vital to this result. Take a bow, Mr Brown.